Vizier of the Menagerie

Oracle Text

You may look at the top card of your library. (You may do this at any time.)

You may cast the top card of your library if it's a creature card.

You may spend mana as though it were mana of any type to cast creature spells.

Card Rulings

4/18/2017 Vizier of the Menagerie lets you look at the top card of your library whenever you want (with one restriction—see below), even if you don’t have priority. This action doesn’t use the stack. Knowing what that card is becomes part of the information you have access to, just like you can look at the cards in your hand.

4/18/2017 If the top card of your library changes while you’re casting a spell or activating an ability, you can’t look at the new top card until you finish casting that spell or activating that ability. This means that if you cast the top card of your library, you can’t look at the next one until you’re done paying for that spell.

4/18/2017 Normally, Vizier of the Menagerie allows you to cast the top card of your library if it’s a creature card, it’s your main phase, and the stack is empty. If that creature card has flash, you’ll be able to cast it any time you could cast an instant, even on an opponent’s turn.

4/18/2017 You may spend mana as though it were mana of any type to cast any creature spell, not just creature spells that you cast from the top of your library.

4/18/2017 You’ll still pay all costs for that spell, including additional costs. You may also pay alternative costs such as emerge or that of As Foretold.

4/18/2017 The top card of your library isn’t in your hand, so you can’t cycle it, discard it, or activate any of its activated abilities.

If you want I can provide the citation for you about moving the card/spell to the stack as part of casting it. As per your question, yes, you do have to reveal the card from the top of your library if you elect to cast it. Hope this answers your question mate.

Once again im reminded that for some reason wizards made snakefolk and naga two different races. Not like i play snake tribal edh, but if i ever want to, id rather have menagerie vizier as an actual snek.
Even cats are snakes on amonkhet, why not this card?

Nagas are evolved snake-like/serpentine entities. It'd be like asking why Humans in MTG aren't also Chimpanzees. From a tribal perspective, you'd totally want a Naga heading the group in EDH. You'd basically have NoReallyAnActualZoo.dec